1. Field of the Invention
This invention relates generally to apparatus for electrically investigating a medium, and particularly but not exclusively to such apparatus for electrical investigation of a borehole penetrating an earth formation.
2. Description of Prior Art
Electrical borehole logging is well known and various devices and techniques have been described. In U.S. Pat. Specification No. 4,468,623, a tool and technique are described for making fine detailed resistivity investigations from which a fine display can be formed in a manner such as that shown and described in U.S. Pat. Specification No. 4,567,759. The tool has a plurality of pads to be pressed against the borehole wall. Each pad carries an array of small electrodes embedded in a conductive pad surface, and measurements of current from the electrodes are recorded in order to detect resistivity anomalies indicative of thin fractures in the earth formation and the stratigraphy of the borehole wall. The conductive pad surface and the electrodes are electrically energised at substantially the same potential. The measure electrodes are electrically insulated from the conductive pad surface with thin insulators and are mounted flush on each pad, providing a plane for contact with the medium to be measured, the plane being maintained at a nearly uniform potential.
Whilst apparatus of this type can be operated satisfactorily in practice, the construction of the pads with arrays of embedded, insulated electrodes is complex and costly. Moreover the ideal concept of a continuous voltage plane is never achieved by such a construction due to discontinuities across the surface caused by the discrete insulated areas.